Engels is perhaps the most neglected, and certainly the most unfashionable, of the major socialist thinkers. Yet many of the most problematical aspects of Marxist theory, such as dialectics, materialism, base and superstructure, scientific socialism and gender, are dealt with most explicitly in the classic texts of Marxism by Engels rather than by Marx himself. This work is not an account of Engels' life. Rather, it offers an interpretation of Engels' social theory, politics and philosophy. Its purpose is to assess Engels' contribution to the genesis of Marxism in the period before 1848; to ask how far Engels departed from this paradigm in the years after 1848; and to examines the degree to which Marx himself shared Engels' intellectual trajectory. The author's aim has not been to defend Engels, but neither has it been to debunk him. Rather, it has been to engage with his thought - and with the justifications and criticsims of it offered by more recent writers - as a means of assessing the strengths and weaknesses of historical materialism, of dialectical materialism, and of scientific and critical Marxism.I write not as a Marxist, nor as a professional Marxologist, but rather as an historian who wishes to arrive at a workable social theory which will generate hypotheses which can be tested against empirical evidence.